Juniper to Cisco Redundancy

I am working on a redundancy project. I'm not 100% sure this is going to work. Here's what is being proposed:

Stacked pair of Juniper EX each with a single member LACP setup in a redundant trunk group to two Cisco 6509s using STP. The Junipers do not participate in the STP.

I see the redundancy working on each side, but I just don't know if the other side will switch unless a physical link is seen.